Hello Anaximedes,
thank you for the comments, please keep providing feedback as we would love to hear what the community think about the tool.
The web viewer mentioned was also made by Animech and is available here GitHub - ue4plugins/GLTFWebViewer
We do not advertise it much as we would prefer users to limit themselves to export common extensions that will work on all major viewers over locking themselves in using one custom viewer.
The plan is that as new Khronos extensions are added that could replace the custom Epic one the exporter would be updated to benefit from that.
Bests.